The IP Multimedia Subsystem (IMS), as defined by the Third Generation Partnership Project (3GPP), merges telephony and Internet technology by providing an all-IP based architecture for the telecommunications industry. The IMS is based on the Session Initiation Protocol (SIP) and makes use of the protocols defined within the Internet Engineering Task Force (IETF). The system offers a network of servers and databases that assist a user agent with the task of establishing and managing sessions.
Various 3GPP standards describe the architecture for IMS roaming in mobile networks. Hereby the infrastructure is split into entities within the Visited Public Land Mobile Network (VPLMN) such as Radio Access, Packet Core Network, Policy and Charging Rules Function (PCRF) of the VPLMN, IP Multimedia Subsystem (IMS), Proxy Call Session Control Function (P-CSCF) and entities within the Home Public Land Mobile Network (HPLMN) such as (IP Multimedia Subsystem (IMS)—Home Subscriber Server (HSS), Policy and Charging Rules Function (PCRF) of the HPLMN, Interrogating/Serving Call Session Control Function (I/S-CSCF), IP Multimedia Subsystem (IMS) Application Servers (e.g. application server which provides the MMTEL service for VoIP).
Regarding roaming functionality, the standards define that the user plane (RTP/RTCP VoIP traffic) might be routed to the HPLMN (home routing) or to the target network (local break out) (cf. 3GPP TR 23.228 Annex M). Additionally, the standards define methods to anchor the media within VPLMN and apply a local PS to CS handover for an inbound roaming user without impact on the HPLMN service domain or the remote user (cf. 3GPP TR 23.856).
The session set-up of a roaming user towards the remote end (so-called mobile originated situation) might be processed involving so-called Home Routing or involving so-called Local Break Out.
Home Routing means that the IP Multimedia Subsystem (IMS) signalling and the associated user plane is routed from the VPLMN via the HPLMN IP Multimedia Subsystem (IMS) domain. The HPLMN applies standard functionalities to terminate the session towards the target User Equipment (UE) (UE B-party) within its own access network or foreign networks (interconnection). The use of Home Routing results in inefficient routing of the user plane with increased resource consumption (IP networks, equipment) and IP transmission delays which jeopardises the quality for e.g. VoIP and Video sessions.
Local Break Out according to 3GPP TR 23.228 Annex M means that the IP Multimedia Subsystem (IMS) signalling is routed from the VPLMN via the HPLMN IP Multimedia Subsystem (IMS) domain towards the remote User Equipment (UE) (UE B-party) within its own or foreign networks, whereas the user plan is directly routed between IP addresses/ports of the VPLMN and the target network. The use of Local Break Out is not applicable if the called user is served by a legacy Circuit Switched (CS) network. Furthermore, the use of Local Break Out results in the challenge that the target network or a transit network between the VPLMN or the HPLMN on the one side and target networks on the other side receives signalling and user plane independently from different networks. This causes problems related to:                correlation,        security (as it is not evident whether the incoming user plane is really associated with the incoming signalling)        accounting between the terminating network and the other involved networks (as it is unclear whether the terminating network will receive fees from the network which provides the signalling or from the network which provides the user plane),        accounting between VPLMN and HPLMN (as the VPLMN, even though being aware about the target IP address to which it routes the user plane, can not apply the target IP address as criteria for accounting the target country/network (CC, NDC) as applied for legacy CS roaming),        quality insurance between the multiple networks (HPLMN, VPLMN, target network, transit networks), as well as        local number portability (as the local numbering schema and/or the portability status at remote side might be unknown to the HPLMN).        
Similar problems arise when a HPLMN IMS domain terminates an incoming media session towards a roaming user (so-called mobile terminating situation).
Sending the signalling (SIP INVITE) from HPLMN Serving-Call State Control Function (S-CSCF) directly to VPLMN Proxy-Call State Control Function (P-CSCF) based on the Proxy-Call State Control Function (P-CSCF) IP address with user identity provided by the SIP INVITE request line and the media from HPLMN Media Gateway (MGW) to VPLMN Media Gateway (MGW) based on the IP address of the Media Gateway (MGW) associated to the Proxy-Call State Control Function (P-CSCF) results in independent IP routing for SIP signalling and media which does not allow routing decisions within transit networks according to the SIP INVITE request line, but allows to apply IP routing only based on the Proxy-Call State Control Function (P-CSCF) IP address. This means that signalling data and media data are routed independent from each other. Transit networks can not apply routing and accounting for, e.g., a VoIP or Video session by taking into account associated SIP and media streams, but will provide IP routing only, which means that they can apply IP peering only, which is different from inter-connection for media sessions.